Infinite BidirectionalRange?

[Ali Çehreli]

Andrei Alexandrescu wrote:
 > On 12/24/10 4:19 PM, Ali Çehreli wrote:
 >> isRandomAccessRange at
 >>
 >> http://digitalmars.com/d/2.0/phobos/std_range.html#isRandomAccessRange
 >>
 >> describes what a RandomAccessRange is:
 >>
 >> <quote>
 >>
 >> A random-access range is a bidirectional range that also offers the
 >> primitive opIndex, OR an infinite forward range that offers opIndex. In
 >> either case, the range must either offer length or be infinite. The
 >> following code should compile for any random-access range.
 >>
 >> R r;
 >> static assert(isForwardRange!(R)); // range is forward
 >> static assert(isBidirectionalRange!(R) || isInfinite!(R));
 >> // range is bidirectional or infinite
 >> auto e = r[1]; // can index
 >>
 >> </quote>
 >>
 >> The part that starts with "In either case" does not make sense to me
 >> (and the sample code does not cover all possible cases). It seems to
 >> suggest that a RandomAccessRange may be an infinite BidirectionalRange.
 >>
 >> Since a BidirectionalRange defines both front() and back(), its being
 >> infinite can only come from asymptoting at one or more points in between
 >> the two ends. Is that useful?
 >>
 >> Does the document need correction or my understanding? :)
 >>
 >> Ali
 >
 > The document I think. Essentially I meant to define the conceptual
 > hierarchy as in this figure:
 >
 > 
http://ptgmedia.pearsoncmg.com/images/art_alexandrescu3_iterators/elementLinks/alexandrescu3_fig03.jpg 


As I've been playing with ranges recently I kept thinking that 
DoubleEndedRange fit better than BidirectionalRange. I wonder why it 
ended up being named BidirectionalRange.

 > and as described in the associated article
 > http://www.informit.com/articles/printerfriendly.aspx?p=1407357
 >
 > So what would be a better formulation?

The spec page sounds as if all of the following make a 
RandomAccessRange, but two of them don't make sense:

a) BidirectionalRange + opIndex + length

b) infinite BidirectionalRange + opIndex (doesn't sound useful; although 
Simen gives a plausible example)

c) infinite ForwardRange + opIndex

d) infinite ForwardRange + opIndex + length (doesn't make sense)

Would the following proposed definition be correct?

<proposed>

A random-access range is a bidirectional range OR an infinite forward 
range that offers the primitive opIndex.

</proposed>

I can't be sure whether the former should also provide the primitive 
'length'. That would make sense, but having the hasLength helper makes 
it also feel like 'length' is an optional property of ranges.

Ali